1. Field of the Invention
This invention relates to a semiconductor device and a method of manufacturing the same.
2. Related Background Art
In recent years, the demand for high performance micro-silicon devices has been remarkably increasing. Such devices typically include electrically writable/erasable nonvolatile semiconductor memories (EEPROMs), logical devices and thin film transistors. Of these devices, nonvolatile semiconductor memories and logical devices require reducing the film thickness of the gate insulating film in order to be downsized without sacrificing their performances. However, conventional silicon oxide film and silicon nitride film intrinsically accompany the problem of increased leak current as tradeoff for the reduction of film thickness. It is difficult for such films to be further downsized and, at the same time, show improved performances.
In view of this problem, there have been developed techniques for using high dielectric film typically made of titanium oxide, zirconium oxide or hafnium oxide that shows a dielectric constant higher than silicon oxide film or silicon nitride film as gate insulating film for the purpose of reducing the leak current and achieving a high insulation effect by increasing the actual film thickness, while maintaining the film thickness reduced to oxide film. However, there is still a strong demand for a higher insulation effect.
Furthermore, there have been developed techniques for regulating the work function and reducing the resistance of such devices by using polycrystalline silicon for the gate electrode and doping it with p type and n type impurities. However, there is still a strong demand for reducing the resistance of the gate electrode and other components of the device.
Meanwhile, in the case of thin film transistors that are typically used for liquid crystal devices, there is a demand for devices having a channel silicon layer that shows increased carrier mobility and is capable of operating with an enhanced level of reliability in order to provide improved functions and performances and high speed operations. Efforts are being paid to poly-crystallize the channel silicon layer and control the crystal grain size thereof by optimizing the channel silicon layer forming conditions including the film forming conditions and the annealing conditions in order to meet the demand. However, there is still a strong demand for devices whose channel silicon layer shows improved carrier mobility and operates with an enhanced level of reliability.
Also efforts are being paid to develop transistors using a single crystal silicon channel formed on an insulating film referred to as SOI (silicon on insulator) substrate in order to realize high speed operations of semiconductor devices.
In the case of an SOI substrate, by a known method, oxygen ions are injected into the silicon substrate by applying a high acceleration voltage from the surface thereof and then the substrate is annealed at high temperature so as to produce an SiO2 layer in the middle of the substrate. However, defects can be produced on the surface silicon film with this method. There is also a known method by which a silicon substrate provided with a silicon oxide film and an ordinary silicon substrate are bonded together with the silicon oxide film sandwiched by the two substrates and the surface of one of the substrates is polished to produce an SOI substrate. However, this method involves a large number of manufacturing steps to raise the cost of the produced substrate.
As pointed out above, there is still a strong demand for gate insulating films showing an improved insulating effect, gate electrodes with reduced resistance and channel silicon layers showing improved carrier mobility and an enhanced level of reliability.
In view of the above identified circumstances, it is therefore an object of the present invention to provide a semiconductor device and a method of manufacturing the same that can improve the insulating effect of the gate insulating film, reduce the resistance of the gate electrode and improve the carrier mobility and the reliability of the channel silicon layer.
Another object of the present invention is to provide a semiconductor device and a method of manufacturing the same by using a substrate that shows a SOI structure with a defect-free silicon layer and can be produced with a reduced number of manufacturing steps.